There is increasing evidence for a role for EGF receptor (EGFR; HER1) activation in recovery from acute renal injury. Expression of heparin-binding epidermal growth factor (HB-EGF), which signals through EGFR as well as HER4, increases in the mammalian kidney in response to acute injury. The precursor for soluble HB-EGF is membrane-associated HB-EGF (proHB-EGF). We hypothesize that in renal epithelial cells, proHB-EGF mediates different cellular functions than soluble HB-EGF. We propose that proHBEGF helps to maintain epithelial cell polarity, integrity and differentiation by: 1) juxtacrine signaling through EGFR and/or HER4; and 2) serving as a scaffold for membrane-associated and cytoskeletal elements (tetraspanins, integrins and glycoproteins) that promote cell-cell interactions. Therefore, cleavage of proHB-EGF may lead to cell activation by release of soluble HB-EGF that can serve as an autocrine and paracrine growth factor as well as by disruption of cell-cell, cell-ECM and cytoskeletal interactions of proHB-EGF and associated proteins that predispose the epithelial cells to motility, proliferation and dedifferentiation. Specific Aim #1 will study the role of juxtacrine activation by proHBEGF in mediation of epithelial cell-cell interactions. Specific Aim #2 will determine the role of proHBEGF's interactions with the tetraspanin-integrin web and with heparin sulfate-containing glycoproteins in mediation of cell-matrix and cell-cell interactions. Specific Aim #3 will investigate the mechanisms by which proHB-EGF protects renal epithelial cells against apoptosis. For Specific Aims #1-3, we will utilize mutations and chimeras of the proHB-EGF molecule that will discriminate among EGFR activation, heparin binding, tetraspanin interaction and cytosolic protein interactions. Specific Aim #4 will utilize crosses of mice expressing floxed HB-EGF with mice expressing targeted nephron segment specific Cre recombinase, as well as transgenic mice overexpressing HB-EGF in proximal nephron, to examine the effects of altered HB-EGF expression on kidney responses to acute and progressive injury.